#canvas{
    cursor: crosshair;
}
button{
    width: 80px;
}
.yellowBtn{
    background-color: yellow;
}
.redBtn{
    background-color: red;
}
.blueBtn{
    background-color: blue;
}
.greenBtn{
    background-color: green;
}
.whiteBtn{
    background-color: white;
}
.blackBtn{
    background-color:black;
}

    <canvas id="canvas" width="900" height="400">浏览器不支持html5</canvas> 
    <br>
    <button id="yellow" class="yellowBtn" onclick="lineColor=‘yellow‘;">YELLOW</button>
    <button id="red" class="redBtn" onclick="lineColor=‘red‘;">RED</button>
    <button id="blue" class="blueBtn" onclick="lineColor=‘blue‘;">BLUE</button>
    <button id="green" class="greenBtn" onclick="lineColor=‘green‘;">GREEN</button>
    <button id="white" class="whiteBtn" onclick="lineColor=‘white‘;">WHITE</button>
    <button id="black" class="blackBtn" onclick="lineColor=‘black‘;">BLACK</button>
    <br>
    <button class="lineWeight4" class="whiteBtn" onclick="lineWeight=2;">2px</button>
    <button class="lineWeight4" class="whiteBtn" onclick="lineWeight=4;">4px</button>
    <button class="lineWeight8" class="whiteBtn" onclick="lineWeight=8;">8px</button>
    <button class="lineWeight12" class="whiteBtn" onclick="lineWeight=12;">12px</button>

   <script type="text/javascript">
    var canvas = document.getElementById("canvas");
    
    //判断是否支持canvas
    if(!canvas || !canvas.getContext){
        return false;
    }
    
    //获得context对象，目前只支持2d
    var ctx = canvas.getContext("2d");
    

    
    //画出画板背景，此处为矩形
    ctx.fillStyle = "black";
    ctx.fillRect(0, 0, 900, 400);
    
    //初始化鼠标是否按下和坐标点位置, true为按下
    var mouseState = false,
        oldX = -10,
        oldY = -10,
        lineColor = "white",
        lineWeight = 2;
        
    //canvas添加鼠标事件, 鼠标移动、鼠标按下和鼠标弹起
    if(window.addEventListener){
        canvas.addEventListener("mousemove", draw, true);
        canvas.addEventListener("mousedown", down, false);
        canvas.addEventListener("mouseup", up, false);

    }else{
        canvas.attachEvent("onmousemove", draw);
        canvas.attachEvent("onmousedown", down);
        canvas.attachEvent("onmouseup", up);
    }
    
    //鼠标按下，拖动画图
    function draw(event){
        if(mouseState === true){
            var newX = event.clientX - 10;
            var newY = event.clientY - 10;
            
            ctx.beginPath();
            ctx.moveTo(oldX, oldY);
            ctx.lineTo(newX, newY);
            
            ctx.strokeStyle = lineColor;
            ctx.lineWidth = lineWeight;
            ctx.lineCap = "round";
            ctx.stroke();
            
            oldX = newX;
            oldY = newY;
        }
        
    }
    
    function down(event){
        mouseState = true;
        oldX = event.clientX - 10;
        oldY = event.clientY - 10;
    }
    
    function up(){
        mouseState = false;
    }
    
    </script>

    function exportImage(event){
        var imgSrc = canvas.toDataURL("image/png");
        document.getElementById("image").src = imgsrc;
    }